Require ChangeMetas for existing models containing unique_together.

Django Evolution previously stored but otherwise ignored unique_together
entries, meaning that older values could not be trusted to have been
applied to the database It could be wildly inconsistent with what's
actually there.

This change considers an older unique_together to be invalidated,
requiring a ChangeMeta mutation, if from Django Evolution 0.6.x or
older.

There's a new flag stored in the signature for each table called
__unique_together_applied. This is used to indicate whether the
field has been applied to the database (or, in the case of an empty
unique_together, at least considered).

When diffing, two identical unique_together values are considered
to be unequal if the older signature doesn't have this new flag.

Projects with existing unique_together values will need to provide
a ChangeMeta for each table. Samples are shown when using evolve --hint.

This also provides some built-in evolutions for the main affected Django
models.
Rewrite the test suite for better error reporting and maintainability.

The existing docstring-based test suite was challenging to work with.
Whenever there was a failure, it was hard to track down what exactly
failed, and most failures caused all future tests in that docstring to
fail. It was also full of redundant boilerplate.

The test suite now exists as standard TestCase subclasses. There's a
bunch of niceties for keeping tests consistent and clean.

The only test still not converted is the 'signature' test suite, which
will come in a future change. That's pretty stable and not likely to
need much in terms of changes or debugging.
Add support for Django 1.5's index_together property.

Django 1.5 introduced index_together for creating an index that spans
columns without being unique. This provides support for that new
property through ChangeMeta.

Unlike with unique_together, we don't have old signature state to worry
about, so we can safely compare values and assume they're correct
(though the actual code to perform the index_together will still
sanity-check the database before generating error-prone SQL).